Failing to GetProcAddress - c++

I'm having trouble with loading a DLL in my assignment project.
Here's the header file:
I have omitted code that works and is irrelevant to the problem. Basically, hinstLib is not NULL but when the line Filter = (FILTPTR) GetProcAddress(hinstLib, "Filter"); is executed, Filter has no value. To me it seems like it is saying that the DLL has been found but it cannot find the function "Filter" inside the DLL and I have no idea why, albeit I could be wrong. I still haven't got my head around how some of this works.
Here is the DLL:
Any ideas anyone? All help is greatly appreciated!
James

Your specifiers are wrong.
A good, concise way do to this is to use same header to in DLL and APP, defining the export-import interface., which uses macro like this:
#ifdef MY_DLL_EXPORTS
#define MY_DLL_API __declspec(dllexport)
#else
#define MY_DLL_API __declspec(dllimport)
#endif
And declarations:
extern "C" MY_DLL_API int Filter(int* data, int count, const WCHAR* parameterString);
Library's .cpp file would use this header and would define MY_DLL_EXPORTS.
If I understand your code right, you made it so that linker tries to export same function from both modules? ALso, function's prototype should be C-compatible to be actually extern "C"

when the line Filter = (FILTPTR) GetProcAddress(hinstLib, "Filter"); is executed, Filter has no value. To me it seems like it is saying that the DLL has been found but it cannot find the function "Filter" inside the DLL and I have no idea why
The function is likely being exported with a decorated name. You are not specifying a calling convention, so the default is usually __cdecl, which prefixes the function name with an underscore, thus it would be exported as "_Filter" instead. But this is compiler-specific behavior, so double-check your DLL's EXPORTS table with a PE viewer/dumper to see the actual name being exported. You may need to add a .def file to your project to ensure the function is exported as "Filter" as desired.

Related

Reason for "{return 0;}" or "{;}" in library's .h file

I am currently working on a project with some old poorly documented code, and am trying to navigate my way through the various .h files that they have provided complete documentation on the use of all of the functions contained (the definitions are held in .lib and .dll files). One of the .h files has functions the are not only declared, but defined with either simple return statements or empty statements as such
class DLL ExampleLibraryClass {
public:
int exampleGetValue() {return 0;}
void exampleSetValue(Type val) {;}
void exampleActionFxn() {;}
};
These would be functions that I expect to return current variable states or perform certain actions, which is why this puzzles me.
Additionally:
I have used Dependency Walker and found that each function does have a matching definition in a dll.
The Keyword DLL has been defined with
#ifndef _PWSDLL_
# define _PWSDLL_
# define WINCALL _stdcall
# ifdef _USRDLL
# define DLL __declspec(dllexport)
# else
# define DLL __declspec(dllimport)
# endif
#endif
_USRDLL is not defined and therefore DLL is defined as __declspec(dllimport)
My question revolves less about the apparent effect of the empty definitions (which do nothing I suppose, and have already been discussed on SO) and more about why the .h file has been written this way and how to utilize the file. Is this a common or known practice? Will the linker still look for definitions to the function in my linked libraries? Are there other pieces of code that I should look for for more clues? Or perhaps in the broadest sense, how should I respond to this?
Many thanks, and please let me know if you need more information to address this, I am unsure what exactly is important in this matter.
EDIT: Added forgotten return types in example code.
EDIT: Added note about DLL definition.
One scenario where you put this code to some use would be to override the functions. These functions hold some default code and could be overridden later.

Export :DLL linking using c++

I know how to use explicit linking for a dll that doesn't contain class but how can link a dll contains class, example:
class math{
public:
int sum(int,int);
};
to load it:
typedef int(*func)(int,int);
int main{
HINSTANCE hDLL; // Handle to DLL
hDLL = LoadLibrary("math.dll");
func add=(func)GetProcAddress(hDLL, "sum");
add(4,5);
return 0;
}
if i do this it stop working and exit the program , if i remove the class it works
GetProcAddress can load up exported symbols form the DLL, provided you called it with the correct name.
In your case the function is not exported in the first place. And if it was, it is surely not called "sum" but some gibberish with 20-40 characters.
For using DLLs with C++ code you'll want to drop the GetProcAddress way entirely, and rely only on the implib that maps the names fine.
For that you add __declspec(dllexport) to the class (preferably with dllimport at the client), and add your DLL project as project reference. Alternatively add the .lib that was created along with the DLL to the client project.
C++ compilers use name mangling to uniquely distinguish identifiers in your program. This mangling causes the name to be significantly different than the plain identifiers you use in your program. Because of this using GetProcAddress is typically impractical for accessing code written in C++ that resides in a DLL. Instead I recommend using __declspec(dllexport) and __declspec(dllimport) to provide painless access to the code residing in your DLL.
In your DLL's project you will want to add a preprocessor definition with a name such as "EXPORT_CLASSES" or one unique to the DLL. This will be used by your DLL and program to determine if a particular declaration should be exported by the DLL or imported by the program.
dllstuff.h
#ifdef EXPORT_CLASSES
#define IMPORT_EXPORT __declspec(dllexport)
#else
#define IMPORT_EXPORT __declspec(dllimport)
#endif
You will then change the class declaration for math to use this. When the DLL is compiled IMPORT_EXPORT will be equal to __declspec(dllexport) and will instruct the compiler and linker that the definitions for this class should be made publicly available (ie. through the DLL's export table).
mathclass.h
#include "dllstuff.h"
class IMPORT_EXPORT math
{
public:
int sum(int, int);
};
Now all you have to do in your main application is include the mathclass.h any time you want to use the math class. You can now instantiate an instance of math and access it's member functions.
#include "mathclass.h"
int main()
{
math m;
int result = m.sum(1, 2);
}
This of course is just a basic description of the process. There are plenty of articles floating around the web (including SO) with provide much more detailed information.

extern "C" not working as expected

I am trying to hook a Win32 API function. I am making a DLL from which I want to export the function, but I am already failing at the basics. My declaration is as follows:
extern "C" __declspec(dllexport) int WINAPI fnTest(void);
but the exported function name is not "fnTest" - as I would expect - but is "_fnTest#0". I can only make it work when declaring the functions calling convention to __cdecl, which results to an exported name of "fnTest", but since the Win32 calling conection is WINAPI/__stdcall this is not an option.
I am using VS2010. Thanks in advance.
That mangling is part of the __stdcall convention. As the called function has the responsibility to remove the parameters from the stack on return, and removing the wrong amount of data from the stack has disastrous consequences, the number of bytes the parameters take is simply appended to the function name after "#" to let the linker catch potential conflicting definition errors.
Could you explain exactly, how does this pose a problem?
You should use module definition file (.def) instead of __declspec(dllexport).
Just use the following .def file:
EXPORTS
fnTest
If you want to do this you will have to export the functions by ordinal rather than by name using a .DEF file.
stdcall provides a decoration that describes the length of the parameters, in this case #0 since you have no parameters. If you had one parameter it would be #4, and so on.

C++ DLL Export: Decorated/Mangled names

Created basic C++ DLL and exported names using Module Definition file (MyDLL.def).
After compilation I check the exported function names using dumpbin.exe
I expect to see:
SomeFunction
but I see this instead:
SomeFunction = SomeFunction###23mangledstuff#####
Why?
The exported function appears undecorated (especially compared to not using the Module Def file), but what's up with the other stuff?
If I use dumpbin.exe against a DLL from any commercial application, you get the clean:
SomeFunction
and nothing else...
I also tried removing the Module Definition and exporting the names using the "C" style of export, namely:
extern "C" void __declspec(dllexport) SomeFunction();
(Simply using "extern "C" did not create an exported function)
However, this still creates the same output, namely:
SomeFunction = SomeFunction###23mangledstuff#####
I also tried the #define dllexport __declspec(dllexport) option and created a LIB with no problem. However, I don't want to have to provide a LIB file to people using the DLL in their C# application.
It's a plain vanilla C++ DLL (unmanaged code), compiled with C++ nothing but a simple header and code. Without Module Def I get mangled exported functions (I can create a static library and use the LIB no problem. I'm trying to avoid that). If I use extern "C" __declspec(dllexport) OR a Module Definition I get what appears to be an undecorated function name... the only problem is that it is followed by an "=" and what looks like a decorated version of the function. I want to get rid of the stuff after the "=" - or at least understand why it is there.
As it stands, I'm pretty certain that I can call the function from C# using a P/Invoke... I just want to avoid that junk at the end of the "=".
I'm open to suggestions on how to change the project/compiler settings, but I just used the standard Visual Studio DLL template - nothing special.
Instead of using .def file just insert pragma comment like this
#pragma comment(linker, "/EXPORT:SomeFunction=_SomeFunction###23mangledstuff#####")
Edit: Or even easier: Inside the body of the function use
#pragma comment(linker, "/EXPORT:" __FUNCTION__"=" __FUNCDNAME__)
. . . if you have troubles finding the decorated function name. This last pragma can be further reduced with a simple macro definition.
You can get what you want by turning off debug info generation. Project + Properties, Linker, Debugging, Generate Debug Info = No.
Naturally, you only want to do this for the Release build. Where the option is already set that way.
You have to declare the functions as extern "C" if you don't want their names to be mangled.
From experience, be careful if you use __stdcall in your function signature. With __stdcall, the name will remain mangled to some extent (you will find out quickly enough). Apparently, there are two levels of mangling, one the extern "C" deals with at the C++ level, but it does not deal with another level of name mangling caused by __stdcall. The extra mangling is apparently relevant to overloading -- but I am not certain of that.
Even without the mangling, the 32-bit and 64-bit builds name exports differently, even with extern "C". Check it out with DEPENDS.EXE.
This can mean BIG trouble to any client that does a LoadLibrary+GetProcAdress to access your function.
So, on top of all the others use a Module Definition File as follows:
LIBRARY MYDLL
EXPORTS
myFunction=myFunction
Yeap, it's a bit of a pain to maintain, but then how many exported functions do you write a day?
Moreover, I usually change the macros like shown below, since my DLLs export functions not C++ classes and I want them to be callable by most programming environments:
#ifdef WTS_EXPORTS
#define WTS_API(ReturnType) extern "C" __declspec(dllexport) ReturnType WINAPI
#else
#define WTS_API(ReturnType) extern "C" __declspec(dllimport) ReturnType WINAPI
#endif
WTS_API(int) fnWTS(void);
The last line used to confuse VisualAssistX a couple of years ago, I don't know if it properly digests it now :-)
Sorry for replying to an old thread, but what has been marked as the answer did not work for me.
As a number of people have pointed out, the extern "C" decoration is important. Changing the "Project / Properties / Linker / Debugging / Generate debug info" setting made absolutely no difference to the mangled names being generated for me in either Debug or Release build mode.
Setup: VS2005 compiling a Visual C++ Class Library project. I was checking the compiled .dll output with Microsoft's Dependency Walker tool.
Here is an example recipe that worked for me...
In project.h:
#define DllExport extern "C" __declspec( dllexport )
DllExport bool API_Init();
DllExport bool API_Shutdown();
In project.cpp:
#include "project.h"
bool API_Init()
{
return true;
}
bool API_Shutdown()
{
return true;
}
Then being called from C# managed code, class.cs:
using System.Runtime.Interopservices;
namespace Foo
{
public class Project
{
[DllImport("project.dll")]
public static extern bool API_Init();
[DllImport("project.dll")]
public static extern bool API_Shutdown();
}
}
Doing the above prevented the mangled names in both Debug and Release mode, regardless of the Generate debug info setting. Good luck.
I know how many times I've tried forcing function names using code and #pragma's.
And I always end with exactly same thing, using Module-Definition File (*.def) at the end.
And here is the reason:
//---------------------------------------------------------------------------------------------------
// Test cases built using VC2010 - Win32 - Debug / Release << doesn't matter
//---------------------------------------------------------------------------------------------------
// SET: Project > Properties > Linker > Debugging > Generate Debug Info = Yes (/DEBUG)
// || (or, also doesn't matter)
// SET: Project > Properties > Linker > Debugging > Generate Debug Info = No + delete PDB file!
extern "C" __declspec(dllexport) void SetCallback(LPCALLBACK function);
> SetCallback
extern "C" __declspec(dllexport) void __stdcall SetCallback(LPCALLBACK function);
> _SetCallback#4
__declspec(dllexport) void SetCallback(LPCALLBACK function);
> ?SetCallback##YAXP6AXHPADPAX#Z#Z
__declspec(dllexport) void __stdcall SetCallback(LPCALLBACK function);
> ?SetCallback##YGXP6GXHPADPAX#Z#Z
//---------------------------------------------------------------------------------------------------
// this also big is nonsense cause as soon you change your calling convention or add / remove
// extern "C" code won't link anymore.
// doesn't work on other cases
#pragma comment(linker, "/EXPORT:SetCallback")
extern "C" __declspec(dllexport) void SetCallback(LPCALLBACK function);
// doesn't work on other cases
#pragma comment(linker, "/EXPORT:SetCallback=SetCallback")
extern "C" __declspec(dllexport) void SetCallback(LPCALLBACK function);
// doesn't work on other cases / creates alias
#pragma comment(linker, "/EXPORT:SetCallback=_SetCallback#4")
extern "C" __declspec(dllexport) void __stdcall SetCallback(LPCALLBACK function);
// doesn't work on other cases / creates alias
#pragma comment(linker, "/EXPORT:SetCallback=?SetCallback##YAXP6AXHPADPAX#Z#Z")
__declspec(dllexport) void SetCallback(LPCALLBACK function);
// doesn't work on other cases / creates alias
#pragma comment(linker, "/EXPORT:SetCallback=?SetCallback##YGXP6GXHPADPAX#Z#Z")
__declspec(dllexport) void __stdcall SetCallback(LPCALLBACK function);
//---------------------------------------------------------------------------------------------------
// So far only repetable case is using Module-Definition File (*.def) in all possible cases:
EXPORTS
SetCallback
extern "C" __declspec(dllexport) void SetCallback(LPCALLBACK function);
> SetCallback
extern "C" __declspec(dllexport) void __stdcall SetCallback(LPCALLBACK function);
> SetCallback
__declspec(dllexport) void SetCallback(LPCALLBACK function);
> SetCallback
__declspec(dllexport) void __stdcall SetCallback(LPCALLBACK function);
> SetCallback
// And by far this is most acceptable as it will reproduce exactly same exported function name
// using most common compilers. Header is dictating calling convention so not much trouble for
// other sw/ppl trying to build Interop or similar.
I wonder why no one did this, it took me only 10 mins to test all cases.
the SomeFunction###23mangledstuff##### is mangled to give the types and class of the C++ function. The simple exports are functions that are callable from C i.e. are written in C or else are declared extern "C' in C++ code. If is you want a simple interface you have to make the functions you export be use just C types and make them non member functions in the global namespace.
Basically, when you use functions in C++, parts of their names now include their signature and suchlike, in order to facilitate language features like overloading.
If you write a DLL using __declspec(dllexport), then it should also produce a lib. Link to that lib, and you will automatically be linked and the functions registered by the CRT at start-up time (if you remembered to change all your imports to exports). You don't need to know about name mangling if you use this system.
In case it wasn't clear from the hundreds of lines of waffle on the subject of mangled exports. Here's my 2c worth :)
After creating a project called Win32Project2 using VS 2012 and choosing export all symbols in the wizard. You should have 2 files called Win32Project2.cpp and Win32project2.h
Both of those will reference an example exportable variable and an example exported function.
In Win32Project2.h you will have the following:
#ifdef WIN32PROJECT2_EXPORTS
#define WIN32PROJECT2_API __declspec(dllexport)
#else
#define WIN32PROJECT2_API __declspec(dllimport)
#endif
extern WIN32PROJECT2_API int nWin32Project2;
WIN32PROJECT2_API int fnWin32Project2(void);
To unmangle CHANGE the last two lines to extern "C" declarations to:
extern "C" WIN32PROJECT2_API int nWin32Project2;
extern "C" WIN32PROJECT2_API int fnWin32Project2(void);
In Win32Project2.cpp you will also have the following default definitions:
// This is an example of an exported variable
WIN32PROJECT2_API int nWin32Project2=0;
// This is an example of an exported function.
WIN32PROJECT2_API int fnWin32Project2(void)
{
return 42;
}
To unmangle CHANGE THESE TO:
// This is an example of an exported variable
extern "C" WIN32PROJECT2_API int nWin32Project2=0;
// This is an example of an exported function.
extern "C" WIN32PROJECT2_API int fnWin32Project2(void)
{
return 42;
}
Essentially you must use the extern "C" prefix in front of declarations in order to force the linker to produce unmangled C like names.
If you prefer to use mangled names for that bit of extra obfuscation (in case the mangling info is useful to someone somehow) use "dumpbin /exports Win32Project2.dll" from a VC command line to lookup the actual reference names. It will have the form "?fnWind32Project2#[param bytes]#[other info] . There are also other DLL viewing tools around if running a VC command shell doesn't float your boat.
Exactly why MS doesn't default to this convention is a mystery. The actual mangling information means something (like parameter size in bytes and more) which might be useful for validation and debugging but is otherwise guff.
To import the DLL function above into C# project (in this case a basic C# windows application with a form on it containing the button "button1") here's some sample code:
using System.Runtime.InteropServices;
namespace AudioRecApp
{
public partial class Form1 : Form
{
[ DllImport("c:\\Projects\test\Debug\Win32Projects2.dll")]
public static extern int fnWin32Project2();
public Form1()
{
InitializeComponent();
}
private void button1_Click(object sender, EventArgs e)
{
int value;
value = fnWin32Project2();
}
}
}

Overloaded functions in C++ DLL def file

I'm writing a C/C++ DLL and want to export certain functions which I've done before using a .def file like this
LIBRARY "MyLib"
EXPORTS
Foo
Bar
with the code defined as this, for example:
int Foo(int a);
void Bar(int foo);
However, what if I want to declare an overloaded method of Foo() like:
int Foo(int a, int b);
As the def file only has the function name and not the full prototype I can't see how it would handle the overloaded functions. Do you just use the one entry and then specify which overloaded version you want when passing in the properly prototyped function pointer to LoadLibrary() ?
Edit: To be clear, this is on Windows using Visual Studio 2005
Edit: Marked the non-def (__declspec) method as the answer...I know this doesn't actually solve the problem using def files as I wanted, but it seems that there is likely no (official) solution using def files. Will leave the question open, however, in case someone knows something we don't have overloaded functions and def files.
Function overloading is a C++ feature that relies on name mangling (the cryptic function names in the linker error messages).
By writing the mangled names into the def file, I can get my test project to link and run:
LIBRARY "TestDLL"
EXPORTS
?Foo##YAXH#Z
?Foo##YAXHH#Z
seems to work for
void Foo( int x );
void Foo( int x, int y );
So copy the C++ function names from the error message and write them into your def file. However, the real question is: Why do you want to use a def file and not go with __declspec(dllexport) ?
The mangled names are non-portable, I tested with VC++ 2008.
In the code itself, mark the functions you want to export using __declspec(dllexport). For example:
#define DllExport __declspec(dllexport)
int DllExport Foo( int a ) {
// implementation
}
int DllExport Foo( int a, int b ) {
// implementation
}
If you do this, you do not need to list the functions in the .def file.
Alternatively, you may be able to use a default parameter value, like:
int Foo( int a, int b = -1 )
This assumes that there exists a value for b that you can use to indicate that it is unused. If -1 is a legal value for b, or if there isn't or shouldn't be a default, this won't work.
Edit (Adam Haile): Corrected to use __declspec as __dllspec was not correct so I could mark this as the official answer...it was close enough.
Edit (Graeme): Oops - thanks for correcting my typo!
I had a similar issue so I wanted to post on this as well.
Usually using
extern "C" __declspec(dllexport) void Foo();
to export a function name is fine.
It will usually export the name
unmangled without the need for a
.def file. There are, however, some
exceptions like __stdcall functions
and overloaded function names.
If you declare a function to use the
__stdcall convention (as is done for many API functions) then
extern "C" __declspec(dllexport) void __stdcall Foo();
will export a mangled name like
_Foo#4. In this case you may need to explicitly map the exported name
to an internal mangled name.
A. How to export an unmangled name. In a .def file add
----
EXPORTS
; Explicit exports can go here
Foo
-----
This will try to find a "best match" for an internal function Foo and export it. In the case above where there is only
one foo this will create the mapping
Foo = _Foo#4
as can be see via dumpbin /EXPORTS
If you have overloaded a function name then you may need to explicitly say which function you want in the .def file
by specifying a mangled name using the entryname[=internalname] syntax. e.g.
----
EXPORTS
; Explicit exports can go here
Foo=_Foo#4
-----
B. An alternative to .def files is that you can export names "in place" using a #pragma.
#pragma comment(linker, "/export:Foo=_Foo#4")
C. A third alternative is to declare just one version of Foo as extern "C" to be exported unmangled. See here for details.
There is no official way of doing what you want, because the dll interface is a C api.
The compiler itself uses mangled names as a workaround, so you should use name mangling when you don't want to change too much in your code.
There isn't a language or version agnostic way of exporting an overloaded function since the mangling convention can change with each release of the compiler.
This is one reason why most WinXX functions have funny names like *Ex or *2.
Systax for EXPORTS definition is:
entryname[=internalname] [#ordinal [NONAME]] [PRIVATE] [DATA]
entryname is the function or variable name that you want to export. This is required. If the name you export is different from the name in the DLL, specify the export's name in the DLL with internalname.
For example, if your DLL exports a function, func1() and you want it to be used as func2(), you would specify:
EXPORTS
func2=func1
Just see the mangled names (using Dependency walker) and specify your own functions name.
Source: http://msdn.microsoft.com/en-us/library/hyx1zcd3(v=vs.71).aspx
Edit: This works for dynamic DLLs, where we need to use GetProcAddress() to explicitly fetch a functions in Dll.